The present invention relates to semiconductor mask-to-wafer aligners in general and, more particularly to an air bearing piston assembly for such aligners.
The prior art aligners had pistons or lift mechanisms which comprised of either two, oil-lubricated, metal sliding surfaces or cylinders riding on a ball bearing system. These aligners had a greater than 1 lb. force between the mask and the wafer and the friction in the lifter systems did not significantly change the force between the mask and the wafer.
The free body force equation of the piston is: F (applied) = F (mask-to-wafer) + F (friction) + F (weight of piston). It can be seen from this equation that F (mask-to-wafer) remains relatively constant (even if the friction force varies 2-1) as long as F (mask-to-wafer) is much greater than F (friction). As long as F (mask-to-wafer) remains in excess of 1 lb. this is true.
In recent years, as the size of integrated circuits increased, the damage done to the mask by the wafer has become more and more important. It has been found that the damage can be decreased considerably if the force between the mask and the wafer is made very low.
It is accordingly a general object of the present invention to provide an improved system for contacting and leveling the wafer to the mask in a semiconductor mask-to-wafer aligner.
It is a specific object of the invention to produce a microforce air bearing piston assembly for such aligners.
It is another object of the invention to provide an air bearing piston assembly which produces a very light pressure between the wafer and the mask during the leveling, contacting and exposing cycles of the aligner.
It is a feature of the invention that the air bearing piston assembly has a very low friction and can produce a very predictable force.